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SEVENTH FRAMEWORK PROGRAMME

THEME ENERGY.2009.3.2.2

Biowaste as feedstock for 2nd generation

Project acronym: VALORGAS

Project full title: Valorisation of food waste to biogas

Grant agreement no.: 241334

D6.2: Evaluation of the quality, biosecurity and agronomic usefulness of digestates from

different digester trials

Due date of deliverable: Month 37

Actual submission date: Month 42

Project start date: 01/03/2010 Duration: 42 months

Lead contractor for this deliverable

Maa Ja Elintarviketalouden Tutkimuskeskus (MTT)

MTT Agrifood Research Finland

Revision [1]

VALORGAS

Deliverable D6.2

of 37 VALORGAS

D6.2 Evaluation of the quality, biosecurity and agronomic usefulness of digestates from different digester trials

Lead contractor: Maa Ja Elintarviketalouden Tutkimuskeskus (MTT)

MTT Agrifood Research Finland

Prepared by

Jukka Rintala, Elina Tampio, Tapio Salo (MTT)

Bryan Lewens (AnDigestion)Contributors

With assistance from

Filipa Vaz, Constança Correia (Valorsul)

Cristina Cavinato (UNIVE)

David Bolzonella, Cinzia Da Ros (UNIVR)

Ying Jiang, Yue Zhang, Sonia Heaven, Charles Banks (Soton)

Joe Mann, Rebecca Arnold (Greenfinch)

MTT Maa Ja Elintarviketalouden Tutkimuskeskus, H-Building, Jokioinen, FI-

31600, Finland

AnDigestion

AnDigestion Ltd, Biogas Plant Donarbon Site Ely Road, Cambridge, CB25

9PG, UK

Valorsul Valorização e Tratamento de Resíduos Sólidos das Regiões de Lisboa e do

Oeste SA, Plataforma Ribeirinha da CP, Estação de Mercadorias da

Bobadela, S. Joao da Talha, 2696-801, Portugal

UNIVE Dipartimento di Scienze Ambientali, Università Ca' Foscari di Venezia, Calle

Larga S. Marta 2137, 30123 Venezia, Italy

UNIVR Dipartimento Biotecnologie, Università Degli Studi Di Verona, Strada Le

Grazie 15, 37134 Verona, Italy

Soton Faculty of Engineering and the Environment, University of Southampton,

Southampton SO17 1BJ, UK

Greenfinch formerly Greenfinch Ltd (now Biogen), Business Park, Coder Road, Ludlow

SY8 1XE, UK

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D6.2 Evaluation of the quality, biosecurity and agronomic usefulness of digestates from different digester trials 1 Introduction ........................................................................................................................ 4

1.1 From food waste to agricultural fertiliser .................................................................... 4

1.2 Digestate nutrient value ............................................................................................... 5

1.3 Digestate biosecurity ................................................................................................... 6

1.4 Digestate use ............................................................................................................... 7

1.4.1 Pre-treatment ........................................................................................................ 7

1.4.2 Composting .......................................................................................................... 8

1.4.3 Storage ................................................................................................................. 9

1.4.4 Application to land ............................................................................................ 10

1.5 Legislation concerning digestate use in agriculture .................................................. 12

1.5.1 EU ...................................................................................................................... 12

1.5.2 Finland ............................................................................................................... 12

1.5.3 Italy .................................................................................................................... 14

1.5.4 UK ...................................................................................................................... 14

1.5.5 Portugal .............................................................................................................. 18

1.6 Study of digestate samples ........................................................................................ 18

2 Materials and methods ..................................................................................................... 18

2.1 Origin of materials ................................................................................................... 18

2.2 Experimental set-up.................................................................................................. 20

2.2.1 Pot experiments .................................................................................................. 20

2.2.2 Nitrogen mineralisation ..................................................................................... 21

2.3 Analytical methods .................................................................................................... 21

2.3.1 Hygienic quality ................................................................................................ 21

2.3.2 Chemical analyses .............................................................................................. 21

3 Results and discussion ..................................................................................................... 23

3.1 Food waste and digestate characteristics ................................................................... 23

3.2 N mineralisation and pot experiments ....................................................................... 27

3.3 Biosecurity ................................................................................................................ 30

3.3.1 Heavy metals and trace elements ....................................................................... 30

3.3.2 Hygienic quality ................................................................................................. 31

4 Conclusions ...................................................................................................................... 33

References ................................................................................................................................ 35

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D6.2 Evaluation of the quality, biosecurity and agronomic usefulness of digestates from different digester trials

1 Introduction

1.1 From food waste to agricultural fertiliser

The depletion of worldwide mineral fertiliser reserves and the fossil fuel use and emissions

due to fertiliser manufacturing are increasing the need for sustainable fertiliser production

methods. An organic nutrient rich fertiliser can be produced through anaerobic digestion

(AD) without fossil fuels and by producing renewable energy at the same time. Anaerobic

digestion of various organic materials also enhances the nutrient cycle when the digestate is

returned to fields (Figure 1).

Figure 1. Digestate and nutrient cycle in organic fertiliser production from food waste

digestates (modified from Al Seadi & Lukehurst 2012).

Across the world animal manure is used as a fertiliser due to its nitrogen and phosphorus

content, but the raw manure contains low amounts of nutrients easily available to plants, such

as ammonium nitrogen (NH4-N). If the material is anaerobically digested prior to land

application, however, ammonium is formed from the organic nitrogen compounds of the

manure and the plant-available soluble NH4-N concentration will increase (Amon et al. 2006,

Clarkson 1990). The anaerobic digestion treatment can also be used for other substrates such

as food waste (FW) to produce organic fertilisers and renewable energy in the form of biogas.

When food waste collected from households is digested in anaerobic digesters and the

digestate is further used as an organic fertiliser in the fields, the nutrient cycle of food

production will be closed.

Food waste contains sugars, fibres, fats and proteins of which the proteins are nitrogen-based

compounds. During anaerobic digestion organic nitrogen is transformed into water soluble

Nutrients in waste stream

Nutrients in recycle stream

AD feedstock

Livestock

production

AD plant

Digestate

Food and agro

industries

Crop

production

Consumer (domestic/

commercial)

Mineral fertiliser

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and plant available ammonium and the conversion efficiency depends on the nitrogen

concentration and quality of the substrate. Food waste contains also phosphorus, potassium

and trace elements, which are essential to plant growth, and all these elements can be found

in the digestate after digestion. However, food waste can also potentially contain e.g. heavy

metals which may affect the end-use of the digestate. European and national regulations limit

the content of PTEs in digestate, and its application to agricultural land, protecting the long

term health of soils, and surrounding environments.

1.2 Digestate nutrient value

While phosphorus and potassium remain unchanged during anaerobic digestion, the organic

nitrogen is degraded and ammonium nitrogen is formed during digestion. The nitrogen and

other nutrient content of the digestate are dependent on the nutrient content of the feed, thus

the material digested has major effect on the fertiliser quality of the digestate (Haraldsen et al.

2011). When food waste, derived from households and public services (restaurants, canteens,

processing plants), is used as feedstock for the digestion process, the amount of plant

nutrients is usually high due to the composition of the original waste.

In the soil NH4-N is converted to nitrate N (NO3) through nitrification which can lead to N

leaching from the soil to water courses or aquifers. To mitigate such losses digestate needs to

be applied when crops are growing and can take up the nitrogen, and the amounts applied

need to be matched to the crop requirements. In general, typical growth patterns involve high

levels of nitrogen take up in the spring and early summer, limited demand in late summer and

autumn, and none during winter months when there is no or very little growth. The use of

digestate is therefore highly seasonal and as a result significant storage facilities are required.

Total nitrogen (N) in digestate and

livestock slurries (kg/m3)

‘Typical’ slurry values taken from “Fertiliser Manual (RB209)” (courtesy ADAS)

0

2

4

6

8

Food based Manure-based Pig slurry Cattle slurry

kg/m

3

Manure-based

digestate (c .8% dm)

Food-based

digestates (c .4% dm)

Cattle slurry

(6% dm)

Pig slurry

(4% dm)

Figure 2. Total nitrogen concentration in digestates and slurries.

Figures 2 and 3 give an indication of the variation in nutrient levels between digestates and

animal manures. The food-based digestate results were based on the analysis of two Animal

By-Product (ABP) compliant plants taking in protein of animal origin which accounts for the

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high levels of total nitrogen in the food-based digestate. At the other end of the scale typical

cow manure has less than half the nitrogen content of the ABP-compliant AD plants. The

proportion of nitrogen in crop available form can be in excess of 80 % total N in food waste

digestate, whilst it is typically only about 50 % in cow slurry. The presence of this much

available N in food waste based digestate makes it a very useful substitute for mineral

nitrogen but the actual analysis will vary depending on input materials, so the agricultural use

of digestate needs to be based on the analysis at the time of use. Although the phosphate and

potash levels in food based digestate are significantly lower than those found in animal

manures (Figure 3) these are present at high enough levels to make a significant contribution

to crop up take.

0

1

2

3

1 2 3 4

kg

/m3

Manure-based

digestate

Food-based

digestate

Cattle slurry Pig slurry

Total phosphate and potash in

digestate and livestock slurries

0

1

2

3

4

1 2 3 4

kg

/m3

Manure-based

digestate

Food-based

digestate

Cattle slurry Pig slurry

‘Typical’ slurry values taken from “The Fertiliser Manual (RB209)” (Courtesy ADAS)

Phosphate (P2O5)

Potash (K2O)

Figure 3. Total phosphate and potash concentration in digestates and slurries.

There are small quantities of magnesium and sulphur present in food waste digestate, but the

availability of sulphur in digestate is poorly understood; so although digestate application

adds sulphur to soil reserves it is difficult to make any claims as to its value in a growing

crop.

Use of digestate also adds organic matter to the soils, enhancing the soil fertility; and

compared to other organic materials such as animal slurry, digestate is also less

decomposable in the soil. Organic matter in slurries has also been reported to decrease the

leaching of nitrogen in the soil (Tambone et al. 2010).

1.3 Digestate biosecurity

As well as the nutrient value, the biosecurity of the digestate is an essential aspect with

respect to the use of the digestate as an agricultural fertiliser. Biosecurity is related to the

material pH, organic and dry matter content, physical, inorganic or inorganic impurities and

to hygienic quality. Impurities may have effects during the processing to fertiliser products

and during fertiliser distribution to the field (physical contaminants); or impurities can

weaken the plant growth and soil quality (organic and inorganic contaminants) or affect the

after-use of the crop material and animal/human health (organic, inorganic contaminants,

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pathogens). Pathogens usually found in FW as a substrate are of different species e.g.

Salmonella, Listeria, Escerichia Coli, Cambylobacter, Mycobacteria, Clostridia and

Yersinia.

Anaerobic digestion acts as a hygienisation process for pathogens depending on the substrate,

process temperature and hydraulic retention times (HRT) applied. The effect of process

temperature is clear: pathogen inactivation in thermophilic processes (50-55 °C) is calculated

in hours but at mesophilic temperatures (30-38 °C) in days (reviewed by Sahlström 2003).

Usually when food waste is used as a digester substrate the material also has to be pre- or

after-hygienised according to the legislation (70 °C, 1 hour, EU 142/2011). Because the

actual concentration of many pathogens in the studied materials can be very low, the hygienic

quality of material is tested with indicator bacteria. These indicator bacteria, usually found in

great quantities in human and animal intestinal tract, are used to indicate the possible

presence of faecal pathogens (Sahlström 2003). Plant pathogens can also be present in food

waste and as reported by Termorshuizen et al. (2003) after mesophilic digestion the

concentrations of Ralstonia solanacearum, (prokaryote, causal agent on potato brown rot)

and Fusarium oxysporum (fungi, causing wilts and root rots in variety of hosts) were below

detection limits. In the same study, however, the inactivation of fungi Sclerotium cepivorum

(which causes white rot in onions) was not achieved in mesophilic digestion.

Inorganic impurities in the digestate, such as heavy metals, can originate from food

production when soils, crops and animal feeds already contain certain levels of heavy metals.

During household waste collection heavy metal containing batteries, metal containers etc. can

end up in the food waste stream, if not removed during waste pre-treatment steps. During

anaerobic digestion heavy metals remain intact and when the digestate is used as a fertiliser

they end up in the soil. Heavy metals such as nickel, zinc, copper and chromium are also

considered as trace elements and vital for animal and human health in small concentrations. If

the concentration increases over the threshold value, however, these heavy metals are

considered harmful for health and environment (Al Seadi & Lukehurst 2012).

Organic impurities such as PAH- and PCB-compounds, dioxins and furans (PCDD, PCDD/F)

are persistent organic pollutants (POPs) which are only sparingly biodegradable in the natural

environment. These compounds originate e.g. from industrial processes and are usually

connected with the treatment of wastewater, but organic pollutants can be found also in food

wastes and further in food waste digestates. Other organic contaminants present in digestates

are halogenated organic compounds (AOX, adsorbable organically bound halogens), LAS

(linear alkylbenzene sulphonate), DEHP (di(2-ethylhexyl) phthalate) and NPEs

(nonylphenols). Physical contaminants such as plastic and metal pieces or large organic

pieces that cannot be digested in the process can affect the end use of digestate by causing

problems in the spreading machinery and uneven fertiliser application.

1.4 Digestate use

1.4.1 Pre-treatment

Digestate originating from organic waste digestion can often be used as a fertiliser as such

without any pre-treatment. Digestate processing methods can be divided into processes where

the nutrient concentration of the material is increased compared to the original digestate, or

where the aim is to produce separate nutrient-containing mineral fertiliser-like materials.

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Because the water content of the digestate is usually quite high and transportation of the high

water content digestate is inefficient, the first processing method is to increase the solids

content (usually to >25 %, Table 1). To achieve this, digestate can be processed by similar

methods as used in the wastewater treatment industry for treatment of sludges, e.g separating

the solid and liquid fractions by a screw press, belt press or decanter centrifuge. In these

technologies the liquid fraction contains the soluble nitrogen and potassium of the digestate

while phosphorus remains in the solid fraction. The solid fraction can be also further dried,

pelletised or composted to increase transportability and marketing value (Lukehurst et al.

2010).

Thermal treatments can also be used to evaporate water from the digestate by using surplus

energy from combustion of the biogas produced from digestion, external energy or solar

energy. Ammonia volatilisation may be a problem during the drying process. Volatilisation

can be prevented by changing the pH of the material, but this can lead to foaming problems.

Other methods to produce nitrogen-rich liquid fertilisers (mineral fertiliser-like products)

from digestate are membrane technologies (nano- and ultrafiltration) incorporated with

reverse-osmosis; ammonia stripping (see also VALORGAS deliverable D3.6); struvite

precipitation (see also VALORGAS deliverable D4.7); and ion exchange.

Table 1. Nutrient concentrations in separated source-segregated food waste (ss-FW)

digestate, mechanically-recovered organic fraction of municipal solid waste (mr-OFMSW)

digestate and composted FW digestate. Unit Liquid fraction

a Solid fraction

a Compost

b

ss-FW mr-

OFMSW

ss-FW mr-

OFMSW

TS % FM 5.84 6.57 14.7 35.0 N/A

Total-N kg tonne-1

FM 112 48.1 54.7 16.2 9.0-28.0

Soluble-N kg tonne-1

TS 65.1 22.4 23.6 4.76 N/A

Total-P kg tonne-1

TS 11.9 4.52 10.5 3.4 1.8-9.3

Total-K kg tonne-1

TS 46.1 17.5 18 3.89 3.4-23.0

Total-C kg tonne-1

TS N/A N/A N/A N/A 191-470

aValues adopted from Zhang et al. 2012

b Values reviewed in Boldrin et al. 2009

1.4.2 Composting

Anaerobic digestate can also be composted after digestion to produce a compost-like

fertiliser/soil conditioner. Usually composting is applied to the separated solid fraction of the

digestate. The composting process also functions as a hygienisation process, if the

temperature and retention time are high and long enough. Because composting is an aerobic

process bulking material is used to keep the total solids high enough to ensure aeration

through the compost pile. Also forced aeration and different composting techniques (e.g. pile,

tunnel or drum composting) can be applied.

Composting increases the solids content and also the carbon and nitrogen content of the

material depending on the composition of the bulking material (Table 2): the solids content

can increase to over 70 % (Tambone et al. 2010). Compared to digestate the nutrient content

in compost is lower, but the organic matter content is high, which enhances the soil

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amendment capacity of the compost (Tambone et al. 2007). By composting also the

phytotoxicity of the FW digestate has been reported to decrease (Abdullahi et al. 2008). In

Portugal, composting of anaerobic digestates is currently the only way to produce organic

fertiliser material from food waste digestate. At Valorsul, Portugal, the food waste digestate

is first dewatered and then composted in a composting tunnel with forced aeration. After one

week the composting is continued in windrows and finally refined by sieving (Vaz 2013).

1.4.3 Storage

After anaerobic digestion of food waste the digestate has to be stored properly before use.

Storage is essential because the application time of fertilisers according to European

legislation is restricted to cover only the growth season of plants (EEC 676/91). Since AD

plants operate with a steady supply of input materials, digestate storage is a key factor in best

practice in the use of digestate as a fertiliser replacer. Grass is useful crop for digestate

application since frequent harvests by cutting or even grazing lengthen the spreading season.

Grazing can, however, be more problematic due to the 21-day grazing ban (applied in the UK

and in Finland). In the UK, in average years a storage period of 6-7 months production should

be sufficient in a grassland area although if the very wet year 2012 is an indicator of more

extreme weather patterns ahead, even this may not be enough. In all arable areas the

spreading season can be even more concentrated, as crop nitrogen demand for all the main

arable crops is in the spring and early summer. There are some crops such as autumn-sown

oil seed rape where there is a limited requirement for nitrogen later in the year, but the

application rates are much lower. Arable farming tend to be in areas with lower rainfall so

early spreading opportunities are much greater. Storage requirements in these areas could be

up to 9-10 months. In Finland, nitrogen-containing fertilisers cannot be spread between 15

October - 15 April, which means a minimum of 6 months' storage. If the soil is not frozen

and is dry in spring, the fertiliser can be applied by 1 April and in autumn until 15 November.

For grass crops nitrogen fertilisers can be applied only until 15 September.

Digestate should be stored in a manner to prevent gaseous emissions and nutrient losses from

the material. Due to nitrogen and carbon degradation, compounds such as ammonia (NH3),

nitrous oxide (N2O) and methane (CH4) are formed. The levels of produced gaseous

emissions during storage are dependent on the temperature, dry matter content, nitrogen

content, pH, and storage conditions (cover material, surface area) of the digestate (Amon et

al. 2006, Rehn & Müller 2011). Nitrous oxide is formed through ammonia nitrification and

nitrate de-nitrification processes in aerobic conditions (Amon et al. 2006), and ammonia is

formed through volatilisation of ammonium (NH4-N, NH4+). The rate of NH3 formation is

highly dependent on the ammonium concentration of the digestate and the volatilisation can

greatly decrease the NH4-N content of the digestate. Residual methane produced in anaerobic

conditions during the digestate storage, however, is a harmful greenhouse gas (GHG) if

released to atmosphere (Rehl & Müller 2011).

The digestate can be stored in covered storage tanks, lagoons or bags to prevent ammonia

losses to the atmosphere and also residual methane losses from the material. The cover

material of the storage facility can be a membrane, concrete, steel or a floating cover of

straw, clay granules or plastic on top of the liquid surface. If the cover of the digestate storage

is air tight the residual methane potential can be utilised and the GHG emissions during

storage decreased (Clemens et al. 2006). Also N2O emissions can be prevented by covering

the material in a gas-tight manner and storing the digestate in anaerobic conditions. By this

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technique 90 % of the gaseous emissions (CH4, N2O, NH3) can be reduced and 55-100 % of

NH4-N retained in the digestate (Rehl & Müller 2011).

Storage is expensive, however, and can easily be a limiting factor on digestate utilisation, so

usually all spreading opportunities are taken that will not compromise the environment. This

can be a difficult distinction when there are limited storage facilities and an AD plant that is a

continuous process. Retrofitting covers on existing farm stores is expensive and frequently

not practical and new stores fitted with covers are costly in relation to open-topped stores.

There is now interest in covered lagoons as a lower cost option, such as the example shown in

Figure 4. The lagoons are fitted with a plastic cover supported by floats. Rain water is

channelled to a collection point where it can be pumped away as clean water.

Figure 4. Lagoon fitted with floating cover.

1.4.4 Application to land

As during the storage stage of digestate utilisation so also during land application, gaseous

compounds (NH3, CH4, N2O) can be released to the atmosphere, decreasing the digestate

ammonium content and increasing GHG emissions. Volatilisation rates are sensitive to dry

matter content and, when applied to a soil which is reasonably dry, digestate should percolate

into the soil quickly resulting in reduced emissions. Conversely ammonia emissions increase

with pH and ammonia is particularly prone to volatilisation at typical pH levels for digestate.

There has been a considerable amount of work done on estimating losses through

volatilisation, and in a review carried out by Lalor et al. (2012) losses of over 70 % of total

NH4-N (TAN) from slurry have been reported following splash plate application. The extent

of volatilisation is dependent on atmospheric and soil conditions at the time of spreading and

is also heavily influenced by the pH and dry matter of the material being spread.

Volatilisation increases in warm, dry and windy conditions and during application to wet

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soils. Digestate can have three to four times the crop available nitrogen content of slurry, so it

is also important that application rates are accurate and precise. Care therefore has to be taken

in choosing the spreading technique and in assessing application rates to meet crop nitrogen

requirements.

The application technique and the pre-treatment of the digestate affect the material losses

during application. Slurry-like digestates can be spread with same equipment as slurries; with

splash plate, trailing hose, trailing-shoe or injection (Lukehurst et al. 2010). More solid

materials can be spread to the fields with compost or manure spreaders. To ensure even and

accurate spreading of digestate and to reduce ammonia volatilisation during spreading

techniques such as trailing hose, trailing-shoe and injection should be used (Table 2). In all

these techniques the digestate is spread directly on or under the soil surface while the splash

plate spreads the digestate to the air increasing the volatilisation and also other environmental

risks. For this reason the use of splash plates is banned in some countries (Lukehurst et al.

2010). With the trailing-shoe and injection techniques the digestate is applied under the top

soil which also minimises the risk of odours.

Table 2. Characteristics of digestate spreading methods (adopted from Birkmose 2009).

Splash plate Trailing hose Trailing shoe Injection

Distribution of slurry Very uneven Even Even Even

Risk of ammonia volatilisation High Medium Low Low or none

Risk of contamination of crop High Low Low Very low

Risk of wind drift High Minimal after

application

Minimal after

application

No risk

Risk of smell High Medium Low Very low

Spreading capacity High High Low Low

Working width 6-10 m 12-28 m 6-12 m 6-12 m

Mechanical damage on crop None None None High

Cost of application Low Medium Medium High

Amount of slurry visible Most Some Some Very little

In Figure 5 the shallow injection tanker is fitted with a flow meter, and with a skilled operator

good spreading precision can be achieved (actual spread rate variation from planned spread

rate <5 %). Applying digestate with such equipment can typically cost £4.50 m-3

(5.2 € m-3

) in

UK conditions (Nix 2013, ADAS personal communication). Using equivalent ADAS/ Nix

figures shallow injection spreading by trailing hose/shoe would typically cost around £4 m-3

(4.6 € m-3

) and broadcast/splash plate £3 m-3

(3.5 € m-3

). Umbilical spreading costs can range

considerably as a lot of time is involved in setting up and moving pipework between fields,

but where large amounts of digestate are to be spread over significant areas very high

spreading rates can be achieved thus reducing the unit spreading costs.

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Shallow Injection into 2nd Cut Regrowth

Fitted with GPS and flow meter

a) shallow injector

Grazing application with farmer owned

machine

NB. 21 day grazing ban

b) trailing shoe

Figure 5. Different spreading options for digestates: (left), (right).

1.5 Legislation concerning digestate use in agriculture

1.5.1 EU

Digestate and digestate-derived compost use in all of the VALORGAS partner countries

(UK, Finland, Italy and Portugal) is regulated by EU legislation. The most significant

regulations are those concerning animal by-products (EC 1096/2009, EU 142/2011),

fertilisers (EC 2003/2003) and the protection of waters against pollution caused by nitrates

from agricultural sources (676/01/EEC). The Animal By-product regulations (EC 1096/2009)

provide three categories for materials according to the risk to public health and animal health,

and regulate the disposal and use of those materials. Materials from category 3, such as food

and catering waste, can be treated in biogas plants after hygienisation (70 °C, 1 hour) and the

digestate can be used as a fertiliser. Category 2 materials (e.g. Salmonella contaminated

slaughterhouse materials), however, must be pressure sterilised (>133 °C, 20 min) before

digestion. According to the regulation the digestion residue can be placed on the market and

used as organic fertiliser or soil improver. The quality of the digestion residue must meet

hygienic standards; the threshold values for E. coli or Enterococcaceae are 1000 cfu g-1

when

no Salmonella is detected in 25 g sample (EU 142/2011).

The fertiliser regulation (EC 2003/2003) is related to inorganic fertilisers. The regulation

includes inorganic primary, secondary and micro-nutrient fertilisers, their fertiliser types,

marking, labelling and packing. The EU’s nitrate regulation (EEC 676/91) aims to protect

waters against pollution caused by agricultural nitrates. The regulation sets measures for

national action programs concerning nitrogen fertiliser use e.g. soil and climate conditions,

plant requirements. According to the regulation the allowed amount of nitrogen that can be

applied in manure is 170 kg N ha-1

year-1

.

1.5.2 Finland

In Finland digestate use in agriculture is regulated by the Law on fertilisers (539/2006) and

the decree of the Ministry of Agriculture and Forestry (24/11) which together implement the

EU’s Animal By-product and fertiliser regulations. The decree (24/11) defines type names for

fertilisers and soil amendments and sets minimum values for nutrients and also limits for

contaminants such as heavy metals (Table 3), pathogens and plant pathogens in inorganic and

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organic fertilisers and soil amendments. Limits for pathogens in the decree are obtained from

the EU regulation (EU 142/201). For soil amendments the decree defines also limits for the

stability of the material (3 mg CO2-C g-1

VS day-1

for packed materials).

In Finland, the limit in the EU’s Nitrate Directive (EEC 676/01) for total nitrogen in manure

(170 kg N ha-1

year-1

) is also considered as the limiting factor for other organic fertilisers

such as digestate. Limits for soluble nitrogen and phosphorus, however, are controlled by the

Finnish agri-environmental aid program (MAVI 2009) which sets soluble-N limits for a

variety of crops with respect to different soil types which are typical in Finland. The soil

types are also divided between northern and southern Finland. The phosphorus fertilisation

control, however, only takes into consideration the crop type and the soil fertility (7 classes).

According to the environmental-aid program, 100 % of the soluble-P content is taken into

consideration if the organic fertiliser originated from animals. If the fertiliser contains

materials (sludges) from wastewater treatment, 40 % of the total-P content is taken into

consideration.

Table 3. Limits for inorganic and organic contaminants in fertilisers according to the Finnish

and English legislation and Portuguese proposed legislation.

Finland Portugal UK

Class I Class II Class IIA Class III

Heavy metals (mg kg-1

TS)

As 25 N/A N/A N/A N/A N/A

Hg 1 0.7 1.5 3 5 1

Cd 1.5 0.7 1.5 3 5 1.5

Cr 300 100 150 300 400 100

Cu 600 100 200 400 600 200

Pb 100 100 150 300 500 200

Ni 100 50 100 200 200 50

Zn 1500 200 500 1000 1500 400

Organic contaminants (mg kg mg kg-1

TS TS)

AOX N/A 500 N/A

LAS N/A 2600 N/A

DEHP N/A 100 N/A

NPE N/A 50 N/A

PAH N/A 60 N/A

PCB N/A 0.8 N/A

PCDD and PCDF N/A 100a N/A

Other (%TS)

Physical contaminants <0.5b 0.5 1 2 3 0.5

Stones N/A 5 5 5 N/A 8 a ng TE kg

-1 TS

b % FM, including stones

N/A= not available

Deliverable D6.2

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1.5.3 Italy

In Italy, according to D.lgs 152/2006 and 205/2010 (deriving from 2008/98/EC, Waste

Framework Directive) the application of digestate as produced for agriculture purposes is not

allowed since digestate, even if it undergoes biological processing, is still a waste. The only

way to upgrade digestate from waste to product status is to compost it together with green

waste, in order to create a fertiliser. In these conditions, digestate is converted into a product

and is then considered under the Fertiliser legislation (D.lgs 75/2010).

On the other hand digestate produced by the treatment of livestock effluents and energy crops

or wastewater sludge can be used directly for agricultural purposes provided it meets the

specific regulations on nitrogen loadings in Nitrate Vulnerable Zones (NVZ) and on heavy

metals and pathogen content.

1.5.4 UK

In the UK regulation plays a major role in the way digestate is used and indeed in the way

that the AD industry is structured. As a result of devolution, regulation in England and Wales

varies from that in Scotland and Northern Ireland although the various regulations are all

compliant with the relevant overlying European Directives. For the purposes of this report

only regulation in England and Wales is considered.

There are Europe-wide concerns over excess nitrates in fresh water and in England it

estimated that 50-60 % of nitrates arise from agriculture. England has designated just under

60 % of its area as NVZs in order to comply with the European Nitrates Directive (EEC

676/91), with a view to controlling the nitrate levels in the water. Farms within NVZs are

legally obliged to obey a number of rules covering the use of manures and other organic

materials including digestate. NVZ regulations build on the Code of Good Agricultural

Practice (COGAP) (Defra 2009) and are part of the cross-compliance set of requirements that

farmers must meet in order to get their Single Farm Payment (EU area-based payment

scheme). Failure to meet these requirements can result in part or all of the payment being

forfeited.

The most important NVZ regulations relevant to digestate are as follows:

Livestock manure N farm limit: A loading limit of 170 kg ha-1 of total N from

livestock manures (deposited during grazing and by spreading) per calendar year,

averaged across the farmed area. This is calculated by using standard nitrogen

production figures for animals on the holding throughout the year. It is only relevant

to digestate if any of the input materials originate from livestock manures. Many food

waste digestion plants in the UK do not take in manures, in which case this limit does

not apply. If any manures are used then an apportionment must be made, or all the

inputs can be categorised as animal manures. There is a clear incentive for operators

not to include animal manures if they are supplying farms in NVZs.

Organic manure N field limit: A spreading limit of 250 kg ha-1 of total N establishes

a maximum application rate for organic manures – this is based on a rolling year.

Closed period (organic manures): This prohibits the spreading of organic manures

with high available nitrogen content during specified periods. This includes materials

with an ammonium N content greater than 30% of total N and therefore includes

digestates. Storage during the closed period is therefore a necessity.

Deliverable D6.2

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N Max limit: This is a limit of the amount of mineral nitrogen and crop available

nitrogen that can be applied to individual crops in any one year. At present digestate

from non animal manure sources is not covered by this rule but will be included when

the regulations are changed in 2013.

Record Keeping: A number of records have to be kept by the farmer in order to

demonstrate cross-compliance. The use of digestate is included in those records.

There are comprehensive software packages available that cover these requirements.

It is becoming noticeable that on some highly stocked farms soil phosphate levels are rising,

and although food waste digestate generally has low phosphate levels it can be difficult to

justify the application of digestate in some cases. The Environmental Agency (EA) expects

farmers to manage their soils to achieve a soil index of 2 and will raise concerns at higher

levels while at very high levels (Index 5) no application at all should take place.

It has been possible since 2010 to supply digestate as a product. The Department for

Environment, Food and Rural Affairs (Defra) funded the development of the PAS110

standard and a Quality Protocol which sets out the criteria for the production and use of

digestate as a quality product (BSI, 2010). This was developed by the EA and the Waste and

Resources Action Programme (WRAP) in consultation with interested parties including

retailers, processors, operators and regulators. Currently seven UK plants are registered as

compliant on the Biofertiliser Certification Scheme website. Digestate from AD plants

processing food waste that is PAS110/QP compliant will have a pasteurising stage in place

and the limits for the hygienic quality are the same as in the EU legislation. The heavy metal

limits for digestates are the same as those specified for compost in PAS100 (BSI, 2011) and

are set in mg kg-1 dry matter (Table 4). The actual heavy metal amounts applied to land are

very small indeed since annual applications are usually limited either by crop demand for

nitrogen or the COGAP limit of 250 kg N ha-1 year

-1.

As an example of digestate utilisation an AD plant in Holsworthy, UK, is introduced. The

Holsworthy plant has been in operation for over 10 years, and was originally designed to

process mainly animal manures with some additional food waste. Pasteurisation was installed

at the time of construction, not to meet ABP regulations (which were not introduced until

2003). but to address farmer fears of cross contamination of bovine tuberculosis via the cattle

slurry. Since 2008 only food wastes have been processed through the plant to meet all ABP

regulations.

Table 4 gives an analysis of the key nutrient qualities of whole digestate produced at the

Holsworthy. Note the high pH levels, low dry matter and high proportion of potentially crop

available nitrogen. Holsworthy digestate is potentially very attractive to farmers because,

having high levels of ammonium N, it can be used as a replacement for expensive industrially

made mineral nitrogen. Nitrogen in this form has to be carefully managed, however, if it is to

be used by the crop and not lost to the environment where it has the capability to cause harm.

Some AD plants in the UK and elsewhere have their own land on which they can spread

digestate. Compromises can be made on crops to be grown and the proportion of nutrients

from digestate to be used. Holsworthy does have some land of its own but the major

proportion of the digestate is exported to other farmers. The plant has been operating for over

10 years, so local farmers are in general aware of the properties of digestate and reassured

http://www.defra.gov.uk/

http://www.defra.gov.uk/

Deliverable D6.2

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over the safety of using it. The key promotional tool used is word of mouth between farmers.

This works for both good and bad, so if any issues are raised by farmers they need to be dealt

with quickly to avoid adverse comment circulating.

Table 4. Holsworthy digestate analysis. Key constituents. Analysis Value Unit

pH 8.4 -

Dry matter 4.6 %

Total nitrogen* 6.3 kg m-3

Ammonium nitrogen 5.9 kg m-3

Phosphate 1.1 kg m-3

P2O5

Potash 1.4 kg m-3

K2O

Magnesium 0.1 kg m-3

MgO

Sulphur 0.9 kg m-3

SO3

*12 month COGAP application limit 39 m3 ha

-1 (250 kg N ha

-1)

Farmers use digestate as a means of saving money, and the key nutrient they consider is

nitrogen. Digestate is high in available nitrogen and it is the nutrient that when applied has

the greatest visible effect. To comply with both product and 'waste' status rules, recent soil

samples for the land to be spread must be available so that nutrient recommendations can be

made. On the grassland farms that are prevalent around Holsworthy few farmers were

regularly soil testing their land. Soil tests are now routinely carried out as part of the

Andigestion digestate supply service. It is only then that any issues surrounding phosphate

and potash issues are realised. Although rare in the Holsworthy area, farmers with significant

arable areas are likely to be more up to date with soil sampling.

Farmers also want digestate applied at the correct time. Near Holsworthy large areas of

grassland are closed off for silage in the early spring, and digestate will be required for

spreading as soon as the ground is dry enough. Ground temperature is rarely a limiting factor

in this area. Further applications can be applied for further cuts later in the growing season.

Very little digestate is applied on winter cereals in this area since there is no crop requirement

for nitrogen in the autumn when the crops are sown, and opportunities to spread in the spring

on growing crops are limited because ground conditions are rarely suitable due to high

rainfall and heavy soils. The demand for digestate is therefore highly concentrated into the

spring and early summer months.

Table 5 gives an example of putting a value on digestate using nutrient values for mineral

fertilisers, but the value will vary depending on the basis of the valuation. The nutrient values

used are based on the current fertiliser prices from a local merchant for nitrogen, phosphate

and potash (UK). The value of a cubic metre (1 tonne equivalent) is relatively low at £6.9 m-3

and since only the ammonium fraction of the nitrogen is potentially available to the crop a

further deduction is made to allow for this in (B).

Deliverable D6.2

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Table 5. Digestate calculation example.

Digestate Recommendation and Valuation

Digestate spread on grassland in the Holsworthy area.

1st Cut Silage Spreading Rate (tonnes ha

-1) = 24

Digestate spread shallow injection March 24th

Nitrogen

(N)

Phosphate

(P2O5)

Potash

(K2O) Value

A Estimated total nutrients in digestate (kg m-3

) 6.3 1.1 1.4 £6.90

B

Estimated available nutrients in digestate (kg m-

3)

a 5.9 1.1 1.4 £6.55

C Requirement for 1st cut silage (kg ha

-1)

b 120 40 80 £174.87

D

Calculate digestate supply of potentially plant-

available nutrients (kg ha-1

) 142 26 34 £157.24

E

Calculate supply of plant-available N after NH3

and NO3 losses (MANNER-NPKc) (kg ha

-1) 120 26 34 £138.64

F Balance required 0.0 14 46 £36.23

G Total N applied per hectare (kg ha-1

) 151

a. Ammonium N content.

b. Derived from RB209

c. MANNER- NPK

Soil Index P= 2 and K= 2-

Fertiliser Prices as at 1st Jan 2013 %

Price

tonne-1

Price

kg-1

Ammonium Nitrate 34.5 £297 £0.86

Muriate of Potash 60.0 £330 £0.55

Triple Super Phosphate 46.0 £285 £0.62

The example given is for a first cut of grass silage yielding 23 tonnes ha-1

at 25 % dry matter.

Nutrient requirements have been derived from the Fertiliser Manual (Defra 2010) which is a

government-backed industry standard. The value of the nutrients removed is considerable (D)

at nearly £160 ha-1

.

In the past there has been no specific published guidance on how to assess nitrogen losses

from leaching or volatilisation when spreading digestate, although losses could be assessed

using a freely available programme called MANNER (ADAS 2010). No specific data was

available on digestate but reasonable calculations could be made based on pig slurry. At the

time of writing a revised version has been released by ADAS called MANNER- NPK (ADAS

2010) which for the first time includes figures for food based digestate. The software allows

the calculation of crop available nitrogen to be made based on a number of variables

including crop, soil type, application rates, dry matter, rainfall, method of application and soil

and atmospheric conditions at the time of application.

In the example given (E) the digestate is being spread when the crop is starting to grow and

the soil is drying out, so leaching losses are not expected. The estimated losses are from

volatilisation and some de-nitrification. Very small amounts of nitrogen are calculated to

become available for a subsequent crop in the same season and crops in the following season.

The application rate chosen for the example provides all the nitrogen requirements for that

Deliverable D6.2

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crop and demonstrates a shortfall on the crop maintenance requirement for phosphate and

potash.

After allowing for nitrogen losses, but including the small amount of nitrogen that will

become available for subsequent crops, the digestate applied is worth £140 ha-1

. Despite the

losses the application provides nearly 90% by value of the estimated crop off take but the

value of one cubic metre of digestate is only £5.90. The significant costs of managing

digestate before it is applied to the soil can thus easily offset the value of the digestate.

1.5.5 Portugal

In Portugal the digestate from anaerobic digestion is not applied to land because of lack of

legislation concerning digestate use. Prior to application the digestate is first treated in

aerobic composting process. There is, however, a proposal for legislation concerning organic

fertiliser use in agriculture in Portugal which sets limits for digestate-derived compost applied

to soils. In this report the limit values used are from the proposed fertiliser legislation (MEID

2010). According to the proposal, organic fertilisers are divided into 4 classes (I, II, IIA, and

III). Limits for heavy metals, organic and inorganic contaminants (stones and other inert

material) contaminants and hygiene indicators are defined for all classes. The fertilisers are

divided into classes according to their heavy metal concentrations (Table 3).

The proposal for organic fertiliser use in agriculture defines that fertiliser from classes I and

II can be used in agriculture, while materials from class IIA are suitable also for wine, fruit

and olive farms as well as for landscaping and gardening. The annual heavy metal quantity

spread to soils is also defined in the proposal and also the quantity of the fertiliser material.

1.6 Study of digestate samples

The aim of this study was to evaluate the agronomic usefulness and the biosecurity of the

digestate samples derived from the VALORGAS partners (MTT in Finland, Italy, UK and

Portugal). The food wastes which the digesters were fed with were also analysed to see the

possible differences between the feed characteristics.

2 Materials and methods

2.1 Origin of materials

Digestates and food wastes used in this study originated from the UK, Italy and Portugal

(Table 6). MTT’s food waste samples, untreated and autoclaved, were originally from the UK

and were sent to MTT Finland, where the FWs were digested in laboratory CSTRs to produce

digestates MTT1 and MTT2, as described in VALORGAS deliverables D3.2 and D3.3.

MTT1 and MTT2 digestates were combined samples from parallel reactors (R1+R2 and

R3+R4) working at an organic loading rate of 4 kg VS m-3

day-1

. Samples were collected

from the reactor outflow on a daily basis and stored in a refrigerator until the desired amount

was obtained (6 litres, stored for a maximum of 2 weeks at 4 °C). After that, samples were

stored frozen (-20 °C), and thawed before analysis.

Samples from the UK, Italy and Portugal were sent frozen to MTT, Finland, where they were

stored at 4 °C until analysis of the nutrient value, heavy metals and hygienic quality. Before

analysing organic composition the samples were frozen (-20 °C) and then thawed again.

Deliverable D6.2

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Table 6. Origin and background information of food wastes and digestates used in the

evaluation. Food waste (FW), vegetable waste (VW), waste activated sludge (WAS), organic

fraction of municipal solid waste (OFMSW).

Sample Origin Scale Temperature Phase HRT

(day)

OLR Feed

MTT1 UK/MTT Lab-scale Mesophilic 1 58 4.0a FW

MTT2 UK/MTT Lab-scale Mesophilic 1 47 4.0a Autoclaved

FW

England Greenfinch Sub-

commercial

Mesophilic 1 25.7 3.3a Domestic FW

Italy Treviso R1 Pilot Mesophilic 1 16 3.8b VW + WAS

R2 Pilot Mesophilic 1 24 2.3b VW + WAS

R3 Pilot Thermophilic 1 24 2.3b VW + WAS

R4 Pilot Thermophilic 1 16 3.8b VW + WAS

F2 Pilot Thermophilic 2 13 4.8a OFMSW

Portugal Lisbon Full scale Thermophilic 2 24 3.7b OFMSW

a kg VS m

-3 day

-1

b kg COD m

-3 day

-1

As shown in Table 6 digestate samples were collected from different size digesters with

different feedstocks. Samples from Italy were from different anaerobic digesters, samples R1

and R2 from mesophilic reactor and samples R3 and R4 from thermophilic reactors which

were all fed with a combination of vegetable waste (VW) and waste activated sludge (WAS).

Sample F2 was from 2-phase digestion, the second phase of which was fed with effluent from

the first phase (dark fermentation) while the original feed to the process was the organic

fraction of municipal solid waste (OFMSW). The OFMSW from Portugal was material

selectively collected by Valorsul from the Lisbon area.

Prior to analysis FW samples were macerated with a Retch Grindomix GM300 knife mill

(Retch Gmbh, Germany). From the Portuguese FW sample also the non-biodegradable

material (plastic cups, plastic bags etc.) were removed before analysing the total and soluble

nutrients while other analyses (characterisation and hygienic quality) were analysed using the

whole sample. The samples and sample amounts used in each experiment are shown in Table

7.

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Table 7. Samples used in each experiment of the deliverable.

Characterisation N mineralisation Pot experiments Biosecurity

FWs

MTT1 x

x

MTT2 x

x

England x

x

Italy x

x

Portugal x

x

Digestates

MTT1 x x x x

MTT2 x x x x

England x x x x

Italy R1 x

x

Italy R2 x

x

Italy R3 x

x

Italy R4 x x x x

Italy F2 x

x

Portugal x x x x

2.2 Experimental set-up

2.2.1 Pot experiments

Nitrogen availability of digestate samples was tested in pot experiment (Figure 6) with Italian

ryegrass (cv. Fabio). The aim was to apply 1500 mg total N in digestates to 5 litres of sandy

soil. The application levels were calculated according to the pre-samples and later the actual

N analysis of added samples revealed some variation in the applied N levels. Control

treatments were inorganic N applications of 0 to 2000 mg N into the pot at 500 mg N

intervals. Sufficient levels of P, K and micronutrients were applied to each pot to keep N the

only responsive nutrient. Three dry matter yields were harvested at 30, 60 and 160 days after

start of the experiment. The fast harvest was after a longer period as the first pots were

growing slowly under glass in September, then the growing period was continued in a

glasshouse for a further thirty days.

Ryegrass was cut in 2 cm high stubble, fresh weight was measured and the harvested sample

dried at 60 °C after which dry weight was determined. Later samples were milled and

Kjeldahl N concentrations were determined.

Deliverable D6.2

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Figure 6. Ryegrass growth before the first yield (left) and the second yield (right).

2.2.2 Nitrogen mineralisation

N mineralisation tests were run according to ISO 14238 'Determination of nitrogen

mineralisation and nitrification in soils and the influence of chemicals on these processes'.

The planned addition level of digestates was to apply 20 mg N into 100 g of test soil.

According to the available information, 2.2–8.6 g fresh matter was added by different

materials. Incubation was done at 20 °C and soil from individual pots was frozen 0, 4, 20 and

48 days after the start of incubation. Soil samples were extracted with 2 M KCl and analysed

with a Lachat auto-analyzer for ammonium and nitrate N. Each material was tested with three

replicates, and soil inorganic N concentrations were compared against incubated soil without

any added fertiliser product.

2.3 Analytical methods

2.3.1 Hygienic quality

Hygienic quality of the FWs and digestates was analysed using E. coli, other coliforms, total

coliforms, enterococcus and sulphite-reducing clostridia as indicator organisms. Analyses of

coliforms were performed according to Baylis and Patrick (1999) using Harlequin

E.coli/coliform (LabM) culture medium. Enterococcus were determined with KF

streptococcus agar according SFS-EN ISO 7899 (Finnish Standard Association 2000) and

sulphite reducing clostridia with sulfite-iron agar according to SFS-EN 26461 (Finnish

Standard Association, 1993). For the qualitative analyses of Salmonella samples were pre-

enriched in buffered peptone water and incubated in Rappaport-Vassiliadis. Aliquots from the

broth were cultured on Salmonella selective Rambach and xylose-lysine-decarboxylase agars.

If growth was observed colonies were confirmed with triple sugar iron agar, urea-agar and

lysine carboxylase broth (37 °C, 24 h) (ISO, 2002).

2.3.2 Chemical analyses

TS and VS were determined according to SFS 3008 (Finnish Standard Association, 1990).

TKN was analysed by a standard method (AOAC 1990) using a Foss Kjeltec 2400 Analyzer

Unit (Foss Tecator AB, Höganäs, Sweden), with Cu as a catalyst and NH4-N according to

McCullough (1967). For soluble COD analysis samples food wastes were diluted 1:10 with

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distilled water and agitated for 1 hour. After that diluted food waste and raw digestate

samples were centrifuged (3500 rpm, 15 min) after which the supernatant was further

centrifuged (1320 rpm, 10 min) and stored in a freezer, and then thawed before analysing

according to SFS 5504 (Finnish Standard Association 2002a). Total COD was measured by

the open reflux, titrimetric method used by the University of Southampton (modified slightly

from Vienna standard method). VFAs (acetic, propionic, iso-butyric, n-butyric, iso-valeric,

valeric and caproic acids) were determined using a HP 6890 gas chromatograph with an HP

7683 autosampler (Hewlett-Packard, Little Falls, USA) and GC ChemStation Rev. B.03.02

software (column HP-FFAP 10 m x 0.53 x 1.0 mm, carrier gas helium). pH was determined

using a VWR pH100 pH-analyser (VWR International).

Crude protein was analysed by Duma’s method (AOAC 1990) using a Leco FP 428 nitrogen

analyzer (Leco Corp., St. Joseph; MI 49085; USA), where the protein content was

determined by multiplying the N% by a factor of 6.25. Soluble carbohydrates were analysed

according to Somogyi (1945) and fat content with Accredited In-house methods No. 4.21 and

4.22: Determination by Soxcap-Soxtec-Analyzer (AOAC Official Method 920.39). NDF

(Neutral Detergent Fibre) was determined with filtering apparatus according to Van Soest et

al. (1991). ADF (Acid Detergent Fibre) and lignin (permanganate-lignin) were determined

according to Robertson & Van Soest (1981). Hemicellulose content was calculated from the

difference between NDF and ADF, when cellulose content was calculated from the difference

between ADF and lignin.

Trace elements were digested in aqua regia according to SFS ISO 11466 (Finnish Standard

Association 2007). Approximately 1.0 g of sample was boiled in 9.35 ml of aqua regia for 2

hours, transferred into a 100 ml volumetric flask and filtered. After digestion trace elements

(Al, B, Fe, Cu, Cr, Mn, Zn, Ni, Co) were determined with ICP-OES (inductively coupled

plasma emission spectrometry, Thermo Jarrel Ash Iris Advantage), and As, Cd, Pb, Mo with

GFAAS (graphite furnace atomic absorption spectrometry, Varian AA280Z). Mercury was

measured by Mercury Analyzer (Varian M-6000A). The technique is based on cold vapour

atomic absorption spectrometry. Mercury was reduced to elemental form with stannous

chloride solution and the mercury vapour was lead (using nitrogen as carrier gas) into an

absorbance cell for measurement.

Soluble nutrients (N, P, K) were analysed from 1:5 (CEN 13652) and 1:60 water extractions.

Samples were shaken for one hour, and then filtered through a cellulose filter (pore size ~ 8

µm). The concentration of NH4-N, NO3-N and PO4-P, where analysed with a Lachat

autoanalyzer. Soluble total N in water extractions was measured with a Lachat autoanalyzer

after oxidation of organic N into NO3-N in autoclave with peroxodisulfate. Soluble total P

and K of water extracts were measured with ICP-OES. Total-N was measured by Kjeldahl

digestion and total-P with nitric acid digestion as described above.

The measurement of P availability was based on modified Hedley fractionation (Sharpley and

Moyer, 2000, Ylivainio et al. 2008), where the fertiliser product was extracted sequentially

with water, 0.5 M NaHCO3, 0.1 M NaOH and 1 M HCl with a ratio of 1:60. First inorganic P

was determined from the extract and then total P concentration was measured after digestion

with peroxidase in autoclave. Organic P concentration was calculated as the difference

between total and inorganic P.

Deliverable D6.2

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3 Results and discussion

3.1 Food waste and digestate characteristics

Both food wastes and digestates were characterised (Table 8) including the organic

composition of materials (Figures 7 and 8). The TS and VS content of the FWs (MTT1,

MTT2, England, Italy, Portugal) were quite similar, only the VS/TS relationship in the Italian

FW was 15 % lower. The lower organic matter content was also seen in the lower SCOD and

COD values of the Italian sample. Total and ammonium nitrogen content was lowest in the

Portuguese sample, which can be explained by the composition of the sample: the Portuguese

sample was visually detected to include a large amount of plastic material, which has also

increased the VS content of that sample. The NH4-N/TKN ratios were 5-7 % in FWs from

MTT, England and Portugal. In the food waste sample from Italy the NH4-N/TKN ratio was

higher (11 %) and the TKN value was lower (5.74 g kg-1

FM) compared to other FWs. The

SCOD/COD relationship was highest in samples MTT1, MTT2 and England which all

originated from the UK. VFA concentrations in all samples were high, possibly partly due to

the relatively long storage times and freezing and thawing of materials.

The characterisation of digestate samples showed much more variation compared to FWs. pH

values were similar in all samples, but TS and VS contents were notably higher in MTT1 and

MTT2 than in the other samples. It might be that in England, Italy and Portugal there was

some pre- or after-treatment phase connected to the digestion which diluted the digestate (e.g.

wet AD process). MTT1 and MTT2 were digestates from laboratory-scale CSTRs which may

also have had some effect on the results. Sample Italy F2 was obtained from the 2-phase

reactor where the dark fermentation phase had already used the easily available organic

matter, giving a decrease in TS, VS, SCOD and COD. In the Italian samples the effect of

HRT can be seen, where the shorter HRT (16 d, R1, R4) shows slightly higher TS and VS

while the longer HRT (24 d, R2, R3) has lower TS and VS. The TKN and NH4-N contents

also showed great variation between samples, and the lowest values were obtained in the

Italian samples as well as in the MTT2 sample. In the samples from England and Italy F2

almost all nitrogen was in soluble NH4-N form (82-85 %), which might be related to pre-

treatments applied in the process. In other Italian samples (R1-R4) and in the Portuguese

sample 60-80 % of nitrogen was in NH4-N form. In MTT2 the low ammonia content was

previously related to the formation of Maillard compounds during autoclaving (see

VALORGAS D3.3) which affected the ammonification during digestion. The NH4-N/TKN

ratio in MTT1 was 52 %.

The organic composition of FWs and digestates is presented in Figures 7 and 8. In FWs the

protein concentration was alike in all samples while the soluble carbohydrates, fat and fibre

concentrations showed some variation. In the Italian and Portuguese samples the soluble

carbohydrates and fat concentrations were lower but hemicellulose and cellulose

concentrations higher compared to other samples. The variations were likely connected with

the FW composition arising from the food consumption and waste separation habits in each

country.

Deliverable D6.2

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Table 8. Food waste and digestate characteristics after freezing and thawing of materials. Results are averages from triplicate analyses from one

sample. Sample pH TS

(g kg-1 FM)

VS

(g kg-1 FM)

VS/TS (%) NH4-N

(g kg-1 FM)

TKN

(g kg-1 FM)

C-tot

(g kg-1 FM)

C/N SCOD

(g kg-1 FM)

COD

(g kg-1 FM)

SCOD/

COD (%)

VFA

(g kg-1 FM)

Food wastes

MTT1 5.5 247.0 229.9 93.1 0.5 7.8 N/A N/A 114.6 364.4 31.4 3.1

MTT2 5.4 226.4 209.0 92.3 0.4 7.3 N/A N/A 104.2 361.2 28.8 2.2

England 5.0 255.1 232.8 91.3 0.6 8.2 N/A N/A 132.9 444.0 29.9 4.9

Italy 5.4 271.1 208.6 76.9 0.7 6.4 N/A N/A 78.4 339.8 23.1 4.8

Portugal 4.7 287.0 264.3 92.1 0.3 5.7 N/A N/A 69.9 412.5 17.0 5.5

Digestates

MTT1 8.0 68.1 50.2 73.6 4.5 8.7 26.9 3.1 15.4 77.1 20.0 3.3

MTT2 7.6 78.8 63.7 80.9 1.7 7.8 25.9 3.3 18.5 100.3 18.4 1.1

England 8.3 19.9 12.3 61.7 3.9 4.7 6.8 1.5 11.2 21.8 51.4 4.1

Italy R1 6.7 33.7 24.6 73.0 1.1 1.9 12.3 6.6 6.9 25.0 27.8 5.0

Italy R2 7.7 23.3 16.4 70.3 1.0 1.3 8.0 6.3 2.5 12.5 19.7 1.1

Italy R3 8.1 18.6 12.6 67.5 1.3 1.8 6.4 3.6 3.1 7.6 41.1 1.1

Italy R4 7.6 34.2 23.9 69.9 1.7 2.2 13.5 6.1 8.4 26.7 31.5 3.4

Italy F2 8.4 7.2 4.1 57.2 1.2 1.4 1.8 1.3 3.2 5.8 55.6 0.3

Portugal 8.3 32.2 18.9 58.7 3.2 4.5 10.3 2.3 7.3 30.6 23.9 0.3

Deliverable D6.2

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Figure 7. Organic composition of food wastes. Results are averages from triplicate analyses

from one sample.

Figure 8. Organic composition of digestates. Results are averages from triplicate analyses

from one sample.

When the organic composition of FWs was compared with the digestates, it was observed

that soluble carbohydrate and fat concentration in all digestates was decreased due to

digestion. Protein concentration increased as a result of the microbial biomass produced in

the digestion process, while the fibre concentrations showed some variation. When the Italian

samples were compared the hemicellulose, cellulose and lignin were observed to be higher in

thermophilic digestates (Italy R3, R4) compared to the mesophilic (Italy R1, R2). The

Portuguese sample also originated from a thermophilic reactor, but the fibre concentrations

were very close to those from the mesophilic samples MTT1, MTT2 and England. The

cellulose content in the Portuguese FW was very high, over 4 times higher than in other

samples, but the same difference was not seen in the digestate sample. The Italian sample F2

showed remarkably lower values in the case of fibre which is likely related to the 2-phase

reactor performance.

The water soluble nutrient content was determined with 1:5 and 1:60 water extractions from

the digestate samples (Table 9). Overall 1:60 water extraction resulted in higher values but

with a few samples water soluble total nitrogen and NH4-N in the 1:5 extraction resulted

0

100

200

300

400

Solublecarbohydrate

Protein Lignin Hemicellulose Cellulose

g/kg

TS

MTT1MTT2EnglandItalyPortugal

0

100

200

300

400

500

Solublecarbohydrate

Protein Lignin Hemicellulose Cellulose

g/kg

TS

MTT1MTT2EnglandItaly R1Italy R2Italy R3Italy R4Italy F2Portugal

Deliverable D6.2

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higher values. In samples MTT2 and Italy R1, R3, R4 and F2 both NH4-N and tot-N resulted

in slightly higher values with 1:5 extraction while in samples England and Italy R2 tot-N was

slightly higher with 1:5 extraction than with 1:60 extraction. Highest total-N values were

obtained from samples MTT1, England and Portugal where the water soluble nitrogen

concentrations were over 4 g kg-1

FM; while in Italian samples the soluble tot-N remained

under 2 g kg-1

FM due to the low total solids in the samples. The autoclaved MTT2 sample

however, gave 3 kg kg-1

FM of soluble total-N which was under 40 % of the total Kjeldahl

nitrogen from the whole sample (in Table 8). This indicates that during the anaerobic

digestion of the autoclaved FW, the nitrogen containing molecules, proteins, cannot be

solubilised which eventually affects the conversion efficiency of the process leading to

reduces methane yields as was observed in VALORGAS D3.3.

Table 9. 1:5 and 1:60 water soluble nutrients in digestates. All values are concentrations in

water soluble form.

MTT1 MTT2 England Italy R1 Italy R2 Italy R3 Italy R4 Italy F2 Portugal

1:5 water soluble (g kg-1

FM)

soluble

tot-N 5.99 3.01 4.44 1.83 1.32 1.86 2.23 1.57 4.02

NH4-N 4.35 1.94 3.32 1.14 1.00 1.39 1.59 1.22 2.77

NO3-N 0.013 0.011 0.011 0.000 0.002 0.002 0.003 0.003 0.007

PO4-P 0.27 0.14 0.06 0.23 0.19 0.23 0.35 0.01 0.13

tot-P 0.33 0.19 0.11 0.30 0.20 0.23 0.35 0.02 0.15

tot-K 3.24 2.50 1.87 0.47 0.31 0.44 0.58 0.75 1.89

1:60 water soluble (g kg-1

FM)

soluble

tot-N 6.15 2.91 4.20 1.25 1.01 1.37 1.88 1.27 4.40

NH4-N 4.89 1.71 3.78 0.97 0.90 1.20 1.46 1.11 3.14

NO3-N 0.014 0.013 0.012 0.002 0.003 0.004 0.002 0.004 0.008

PO4-P 0.39 0.17 0.10 0.28 0.23 0.27 0.43 0.01 0.17

tot-P 0.47 0.22 0.12 0.35 0.25 0.31 0.45 0.03 0.21

tot-K 3.56 2.54 1.97 0.47 0.32 0.45 0.60 0.77 2.11

The total water soluble phosphorous (tot-P) and phosphate (PO4-P) analysed were

consistently higher with 1:60 extraction than with 1:5 extraction. In sample Italy R4 95 % of

the water soluble total-P consisted of PO4-P while in samples R3 and R3 the PO4-P/tot-P ratio

was 89 and 88 %. The phosphate to tot-P ratio was around 80 % with samples MTT1, MTT2,

England, Italy R1 and Portugal while in Italy F2 the ratio was only 50 % likely due to the 2-

phase process configuration. However, the water soluble potassium showed very similar

results at both extraction ratios.

The plant available P was analysed with Hedleys fractionation (Figure 9). A considerable

proportion of P was water soluble, except in the Portuguese digestate. The sum of water and

NaHCO3 extractable P (Olsen-P) can be considered as plant available and this was 50-70 %

in samples of MTT, Italy and England. When phosphorus is from organic sources it can be

expected to be readily soluble and thus readily available for plants. The lower P solubility of

sample from Portugal suggests that the input is different from the other tested FW digestates.

Deliverable D6.2

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Figure 9. Phosphorus solubility determined with Hedleys fractionation.

3.2 N mineralisation and pot experiments

In the N mineralisation experiment the planned application rate of 200 mg total N kg-1

of soil

varied, as the applied materials had slightly different N concentrations as expected (Table 10).

All materials contained considerable amount of NH4-N. Application of dissolved organic N

(DON) of 1:5 water extractions was 30–60 mg kg-1

soil and this proportion of organic N can

be considered most easily mineralised. Mineralisation of organic N was of the same

magnitude (30 N mg kg-1

soil) from all other digestates except the digestate from England

(Table 11 and Figure 10). Nitrification of NH4-N to NO3-N happened at a fast rate after all

digestate applications (Figure 11). Considering the fertiliser value, the digestate from

England responds to its NH4-N concentration. Other digestates had lower ammonium

concentrations and 15–30% of their organic N mineralised during the incubation.

Figure 10. Nitrogen mineralisation of digestates in the incubation experiment.

0

5

10

15

20

25

MTT1 MTT2 England Italy R4 Portugal

gP/k

gTS

PHCl

PNaOH

PNaHCO3

Pwater16h

Pwater 4h

0

20

40

60

80

100

120

140

160

180

200

0 10 20 30 40 50 60

N m

g/kg

days from start of incubation

MTT_1

MTT_2

England

Portugal

Italy

Deliverable D6.2

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Figure 11. Soil ammonium (NH4-N) and nitrate (NO3-N) concentrations during the

incubation.

Table 10. Applied fresh matter, nitrogen components and mineralisation of organic N in the

incubation test. Application Applied mg kg

-1 Mineralisation from organic N

FM

g per

100g

Total-N Organic-N DON NH4-N NO3-N mg kg-1

% of

DON

% of

organic-N

MTT1 2.2 205 108 36 97 0 36 100 33

MTT2 2.6 171 121 27 50 0 34 125 28

England 4.8 235 77 53 158 1 2 3 2

Portugal 5.1 244 102 64 142 0 29 45 28

Italy R4 8.6 318 181 54 137 0 26 47 14

DON = organic N dissolved in 1:5 water extraction

The N uptake in pot experiments was executed with Italian ryegrass (cv. Fabio) (Figures 12

and 13). Applications of digestate NH4-N, soluble N and total N are shown above each

digestate bar. Blue vertical lines show the respective yield level of inorganic fertiliser

application compared to NH4-N (lower) and soluble N (higher line). Error bars show standard

deviation of the treatments.

All digestate applications produced better yield than could be estimated according to their

NH4-N content. Digestates from England, Italy and Portugal increased ryegrass yield only

slightly more than expected according to the NH4-N content. Soluble N was not fully

available from those digestates. Digestates MTT1 and MTT2 seemed to release all of their

soluble N as yields were comparable to their soluble N content.

-20

0

20

40

60

80

100

120

140

160

180

200

0 20 40 60

N m

g/kg

so

il

days after start of incubation

MTT_1 NH4-N mg/kg

MTT_1 NO3-N mg/kg

MTT_2 NH4-N mg/kg

MTT_2 NO3-N mg/kg

England NH4-N mg/kg

England NO3-N mg/kg

Portugal NH4-N mg/kg

Portugal NO3-N mg/kg

Italy NH4-N mg/kg

Italy NO3-N mg/kg

Deliverable D6.2

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Figure 12. The effect of digestate N on ryegrass dry matter yield in the pot experiment.

Figure 13. Ryegrass N uptake in the two first yields of the pot experiment.

Accumulated ryegrass yields per pot

0

10

20

30

40

50

60

70

80

90

N0 N500 N1000 N1500 N2000 MTT1 MTT2 England Italy Portugal

Dry

mat

ter

yie

ld (

g)

NH4-N 730

Sol. N 1000

tot-N 1540

NH4-N 1190 1030 1070

Sol. N 1580 1440 1550

tot-N 1760 2390 1830

NH4-N 380

Sol. N 580

tot-N 1280

Accumulated N uptake per pot

0

200

400

600

800

1000

1200

1400

1600

N0 N500 N1000 N1500 N2000 MTT1 MTT2 England Italy Portugal

N u

pta

ke (

mg/

po

t)

NH4-N 730

Sol. N 1000

tot-N 1540

NH4-N 1190 1030 1070

Sol. N 1580 1440 1550

tot-N 1760 2390 1830

NH4-N 380

Sol. N 580

tot-N 1280

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3.3 Biosecurity

3.3.1 Heavy metals and trace elements

The metal concentrations in food wastes and digestates were studied from dried samples. In

food wastes, concentrations between samples varied greatly thus e.g. cadmium and mercury

concentrations were low (<0.1 mg kg-1

) in all samples (Table 11). In the autoclaved sample

MTT2 higher concentrations of Pb, Hg, Al, Mo, B, Fe, Cu, Zn, Mn, Cr and Co were observed

compared to sample MTT1, possibly due to residues from the autoclaving apparatus or to the

release of metals during pre-treatment.

Table 11. Metal concentrations in food wastes.

mg kg-1

DM

MTT1 MTT2 England Italy Portugal

Pb 0.20 2.21 0.65 3.35 0.49

Ni 0.64 0.47 1.03 4.21 0.76

Hg 0.06 0.08 0.08 0.09 0.05

Cd 0.06 0.05 0.06 0.10 0.02

Al 100 120 160 3590 140

Mo 0.50 0.63 0.44 1.32 0.20

B 10.1 22.0 8.5 24.4 13.0

As 0.45 0.45 0.39 1.41 0.22

Fe 0.13 7.12 0.35 5.61 0.28

Cu 4.93 8.35 5.74 16.56 9.61

Zn 28.2 37.8 29.4 41.5 93.3

Mn 78.1 154.8 69.9 126.0 8.8

Cr 1.09 3.25 1.76 18.74 1.31

Co 0.14 0.37 0.21 0.85 0.40

From food wastes to digestates the metal concentrations increased which indicated

solubilisation of metals during digestion, and also the reduction in solids content during

digestion. Concentrations of mercury, copper, lead and aluminum were notably higher in the

Italian samples R1-R4 (Table 12) compared to other samples. This might be due to the

different origin of these samples (feed material: VW+WAS). The cadmium concentration was

highest in Italian samples R1-R4 but also in the Portuguese sample (~1 mg kg-1

) while the

highest cobalt concentrations (10-30 mg kg-1

) were observed with samples UK, MTT1 and

MTT2, all originating from the UK.

The usability of the digestates as fertiliser in agriculture was studied by comparing the metal

(Pb, Ni, Hg, Cd, Cu, Zn, Cr) concentrations with the limit values from national legislations.

For trace elements (Al, Mo, B, Se, Fe, Mn, Co) there were no limit values. For arsenic limit

values were implemented only in Finnish legislation. The Portuguese legislation proposal for

compost had the lowest limit values for heavy metals (class I) while in Finland the limiting

values allowed the highest heavy metal concentrations in digestates (Table 3).

Deliverable D6.2

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Table 12. Metal concentrations in digestates.

mg kg-1

DM


Pb 2.10 5.60 5.63 79.67 98.67 99.21 98.04 6.64 11.66

Ni 17.78 16.64 42.39 19.69 24.76 26.50 22.25 10.42 6.69

Hg 0.12 0.19 0.09 1.77 2.49 2.41 1.81 0.16 0.29

Cd 0.23 0.13 0.26 0.88 1.39 1.28 1.11 0.32 1.50

Al 870 440 560 6350 8940 8410 7680 2040 2050

Mo 4.77 3.80 8.39 6.65 8.53 8.58 7.61 3.82 3.30

B 33.7 27.2 34.8 70.0 61.5 68.1 65.7 65.4 55.6

As 0.70 0.44 1.01 1.90 2.99 2.81 2.59 3.09 3.32

Fe 4.52 6.16 2.32 5.42 6.60 6.41 5.91 5.25 3.74

Cu 25.64 22.37 21.69 551.60 512.10 522.60 626.50 34.83 58.70

Zn 116.0 94.6 175.0 800.0 1099.0 1056.0 1006.0 140.0 401.0

Mn 283.7 230.2 77.6 95.9 110.5 109.3 105.3 139.1 105.4

Cr 9.82 11.89 7.48 29.09 37.29 35.72 32.89 23.51 12.97

Co 13.09 11.24 31.28 1.64 2.35 2.51 2.19 0.97 1.45

All the studied digestates were below the limiting values of lead, nickel and chromium.

Samples MTT1, MTT2, UK and Italy F2 had all metal concentrations under the limiting

values appropriate for fertiliser materials in Portugal (if composted after digestion), Finland

and UK. Because Portuguese legislation proposal for compost (class I and II) and digestate

legislation in UK have low limiting values for mercury, cadmium, copper and zinc, samples

Italy R1-R4 were not suitable as fertiliser materials according to these limits. The zinc

concentration in the Portuguese sample exceeded the Portuguese class I limit (200 mg kg-1

)

and was very close to the limit value in UK (400 mg kg-1

). A limit for arsenic was

implemented only in Finnish legislation for organic fertilizers, but none of the samples

exceeded the limit (25 mg kg-1

). It should be noted that the very high degradability of food

waste can give relatively high concentrations of metals in digestate when quoted on a dry

matter or organoc dry matter basis, rather than a wet weight basis, due to the high degree of

solids degradation achieved compared to less degradable wastes.

3.3.2 Hygienic quality

The hygienic quality of the food wastes and digestates was determined with hygiene

indicators E. coli, other coliforms, total coliforms, Enterococcae, Sulphate reducing clostridia

and Salmonella (Figures 14 and 15). Because samples were shipped frozen and were stored

few days in the freezer before analysis, the results cannot be related to results from fresh

samples. However, no Salmonella was detected in neither FW nor digestate samples.

Enterococcae was found in similar concentrations in all FW samples except the autoclaved

MTT2 sample, where the autoclave treatment had sterilised the material very efficiently.

Coliforms were detected in all samples except MTT2.

As in the FW samples also in the digestate samples the concentration of Enterococcae

remained quite high (except the UK sample), and in both MTT samples the concentration was

notably increased compared to the FWs. This phenomenon is likely due to the microbial

population from the inoculum of MTT’s reactors. In UK there is some hygienisation process

prior or after the digestion process because almost no hygiene indicators were found in the

Deliverable D6.2

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sample from England. Italian samples R1-R4 showed diverse hygiene indicator population

which slightly reflected the temperature and retention time applied to each reactor. Other

coliforms were detected in the Italian digestates (R1-R4) but not in the Italian FW: this was

due to the different origin of R1-R4 feed material (WAS+VW) which was not analysed in

this experiment. The Italian F2 sample obtained from the second phase of the 2-phase process

contained only Enterococcae and Clostridia as well as the Portuguese sample, likely due to

the thermophilic conditions applied during the anaerobic digestion.

Figure 14. Hygienic quality of food wastes (after freezing and thawing).

Figure 15. Hygienic quality of digestates (after freezing and thawing).

According to EU regulations concerning animal by-products (EC 1069/2009 and EU

142/2011) and their digestion residues the standards for the threshold values for E. coli or

Enterococcaceae are 1000 cfu g-1

when no Salmonella is detected in 25 g sample. According

to this legislation the samples MTT1, MTT2, England, Italy F2 and Portugal could be used in

agriculture because no Salmonella was detected and the concentration of E. coli remained

under the threshold value. However, Italian samples R1-R4 would not be appropriate for

agricultural use without a hygienisation step.

5.0E+00

5.0E+01

5.0E+02

5.0E+03

5.0E+04

5.0E+05

5.0E+06

5.0E+07

MTT1 MTT2 England Italy Portugal

cfu

/g

E.Coli

Other coliforms

Total coliforms

Enterococcae

Sulphite-reducing clostridia

5.0E+00

5.0E+01

5.0E+02

5.0E+03

5.0E+04

5.0E+05

5.0E+06

5.0E+07


cfu

/g

E.Coli

Other coliforms

Total coliforms

Enterococcae

Sulphite-reducing clostridia

Deliverable D6.2

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4 Conclusions

Food waste treated in anaerobic digesters to produce organic fertiliser for agricultural use

enhances the nutrient cycle in the food production and food waste chain. Food waste

collected from municipal sources may, however, contain impurities which can affect the end-

use of the digestate. The food waste may be contaminated by organic, inorganic or physical

impurities which can affect the fertiliser value, pose human and animal health risks, or

complicate the digestate spreading. FW may also be contaminated with pathogens which

according to EU legislation must be destroyed before or after the anaerobic digestion process

to provide a safe end product.

Use of FW as an organic fertiliser in agriculture is highly controlled by legislation in Europe.

The legislation is based on the EU’s framework directives which are implemented by each

country in their national legislation. There are, however, major differences between countries,

e.g. Finland, UK, Italy and Portugal which were studied in this report. Application of

digestate is allowed in Finland and in the UK, while in Italy and Portugal only composted

digestate is suitable for fertiliser use. Also the implementation of EU guidelines varies, e.g. in

Finland the soluble-N limit per hectare per year (170 kg) is applied to all digestates

irrespective of the origin of the feed material. In UK the limit is applied only to manure-based

digestates, as stated in the EU directive (EEC 676/01). Also the application of phosphorus to

land is regulated differently in each country and the regulations in Finland are dependent on

the feed material of the anaerobic digester. National legislation is essential, however, when it

comes to national and regional climate, soil and crop types, which vary between countries.

The soil and climate conditions affect the applicable N fertiliser capacity and are thus

regulated by each country.

Under the UK waste regulations the EA requires digestate spreading permissions to be in

place well before spreading commences. This can be both wasteful and expensive but it is

vital that permission is in place to make use of all spreading opportunities. This bottleneck is

removed by product status. Digestate has the potential to replace mineral fertiliser nutrients

for nitrogen, phosphate and potash. However the presence of significant quantities of crop-

available nitrogen especially means that digestate has to be used carefully both to give

maximum agricultural benefit and to avoid causing damage to the environment. Processing of

digestate in order to concentrate the nutrients or improve the characteristics of the digestate

would clearly offer major benefits in managing digestate, but the processes would need to be

both economic and reliable. These processing techniques include solid-liquid separation

processes with centrifuges or belt presses. The produced solid fraction can be further

pelletised or used as such, if that is allowed in the current legislation. The solid fraction can

also be composted to produce soil amendment material, if the legislations require this

treatment. The liquid fraction could be used as such, or further processed to produce fertiliser

products with e.g. ammonia stripping or struvite precipitation (see also VALORGAS

deliverables D3.6 and D4.7).

If digestate is used to achieve best agricultural benefit the spreading of digestate is

necessarily highly seasonal. This poses major logistical pressures for AD operators, which

can be mitigated by the use of on-farm storage and spreading facilities. Storage of the

digestate is essential when it comes to limited fertilising seasons, and can be executed with

different technologies e.g. floating covers on top of the storage tanks to prevent gaseous

emissions. Some of the logistical problems could be prevented by using e.g. solid-liquid

Deliverable D6.2

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separation, reducing the amount of transported water and concentrating the nutrients which

also reduces the price for the transport. Application of digestate must be well planned to

prevent environmental problems caused by the volatilisation of NH3 during spreading and to

provide plants with the effective amount of nutrients.

The agricultural usefulness and the biosecurity aspects of digestates from different AD plants

involved in the VALORGAS project were analysed. The results showed that the digestates

studied had potential as fertiliser as such, without any pre-treatment step such as solid-liquid

separation. The characteristics of digestates varied in terms of TS concentration due to

variation in the AD reactor configurations. Thus it would be efficient to pre-treat the digestate

samples from Italy to decrease the water content of the digestates and the applicable amount

per hectare. Italian samples (R1-R4, originating from vegetable waste and WAS) also had

higher heavy metal concentrations making these samples not appropriate for agricultural use

according to the stricter Portuguese (for composted digestate) and UK legislation. However,

digestates from food waste were all suitable fertiliser materials. Also in the hygienic quality

analysis Italian digestate samples were the only ones not to pass the EU regulations on E. coli

and enterococcus concentrations; this, however, could be overcome by a hygienisation step

before or after digestion. All in all, the food waste digestates had good biosecurity in the case

of heavy metals and pathogens.

When the nutrient availability to plants was tested the sample MTT2, derived from FW

generated in UK and pre-treated by autoclaving, showed reduced nitrogen availability which

was connected with the effects of the autoclave treatment on the waste. All digestates studied

showed potential to be used in agriculture with just few adjustments regarding the biosecurity

and transportability of materials.

Deliverable D6.2

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